1. Field of the Invention
The present invention relates to a vent system for fuel storage tanks such as the type employed at petrol stations.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Diesel and petrol storage tanks employed at petrol stations and elsewhere are usually vented to atmosphere to avoid relative vacuum or relative pressure building up as the contents are periodically dispensed to customers' vehicles or replenished by tankers. Vapour recovery systems may be installed, in order to help reduce the volume of petroleum vapour vented to atmosphere during dispensing and/or replenishment, and this includes provision for venting of the tank at least under certain conditions, as discussed further below.
At petrol stations, for reasons of public health and safety, the vent to the atmosphere is provided at a high level. Typically, a number of vent pipes or stacks are provided, usually located away from the main customer refueling area. Diesel storage tanks are usually individually vented whereas more than one petrol storage tank may be connected to one or more vent pipe stacks via a manifold.
The vent pipes are protected at their upper end by vent caps, which serve to keep rain out of the pipes as well as birds, insects, floating debris etc. The vent caps for petrol tanks are also provided with a flame arrestor gauze, but this is not currently required for diesel tank vent caps. Flame arrestors on petrol vents can also double as screen filters for the air intake. In fact, where screen filters have been used on diesel tank vent caps, it has been found that the diesel vapour leaves a sticky residue on the filter element which can trap dirt and block. Therefore the filter would require periodic cleaning. Petrol vapour does not leave a residue, and in fact can have a self-cleaning effect.
In the case of petrol vent pipes on sites with vapour recovery systems, pressure vacuum valves are usually provided, incorporated into the vent cap. A pressure vacuum valve (PVV) is designed to keep the vent pipe closed to atmosphere unless the relative pressure or relative vacuum inside the tank exceeds pre-determined levels. The valve will then open and allow either the petroleum vapour to vent to atmosphere or atmospheric air to enter the vent pipe to control the pressure or vacuum level. In accordance with industry standards in the UK, the pressure limit is typically set to 35 millibars above atmospheric pressure and the vacuum limit is set to 2 millibars below atmospheric pressure. The main purpose of the PVV is to control vent emissions and assist vapour displaced from the tank during tanker deliveries to be pulled back into the ullage space of the tanker rather than being vented to atmosphere. Recovery of petrol vapour is an economic benefit as the vapour can be reconverted into fuel, as well as reducing emissions harmful to the environment. Diesel vapour however is not currently recovered during tanker deliveries, so PVVs are not usually employed on diesel vent pipes. If they were, they would suffer from similar diesel vapour residue issues as described above in relation to filter elements.
It will be necessary periodically to check and service the vent caps and PVVs. However, vent pipes are usually about 6 meters in height. Regulations governing working at height prevent the use of ladders and therefore Petrol Station Operators or Contractors are required to use mobile access platforms (“cherry pickers”) or other approved means of accessing the tops of the vent pipes. This procedure is relatively time-consuming and expensive, and also takes up space on site which adds to operational difficulties.